Increasing demands will be placed on wireless communication speed e.g. due to the fact that wireless communication devices are used for other purposes than for transmitting mere speech. Wireless communication devices will be used e.g. for file transfer, browsing of www pages with a www browser, browsing of WAP pages with a WAP browser, transmission of real time video signal, etc. In such applications, relatively high speed communication is required to avoid unreasonable waiting times.
In a prior art second generation GSM system, data is transmitted in compliance with the TDMA principle (time division multiple access), where eight timeslots of one TDMA frame are allocated for different wireless communication devices. A maximum of one reception and one transmission timeslot is allocated for one wireless communication device. Consequently, the communication rate of the wireless connection is limited to approximately 14.4 kbit/s, at most. To increase the communication speed, a high speed circuit switched data connection has been designed, which allows, if necessary, more than one TDMA timeslot to be allocated for a single wireless communication device for reception and/or transmission (multi slot). Additionally, the general packet radio service GPRS, designed for the GSM system, uses multiple timeslots and enables higher communication speeds compared to second generation GSM systems. Such mobile communication systems are also being developed where a wireless communication device can simultaneously communicate using both a circuit switched connection and a packet switched connection. Hence, some of the transmission timeslots allocated for the wireless communication device are used in the transmission of the circuit switched connection, and some of the transmission timeslots are used in the transmission of the packet switched connection.
In the GSM system, bursts are used in the transmission of information in a manner that during one timeslot, one burst is transmitted. Such a burst contains, in addition to the information to be transmitted, e.g. acquisition bits (training sequence) and tail bits. Speech and data is transmitted in these bursts as follows. The information to be transmitted is converted, if necessary, to binary form and divided into data frames, each having a certain amount of bits. These data frames are also called radio blocks. These data frame bits are typically placed in more than one burst. The division into bursts is carried out e.g in a manner that the data frame bits are divided into eight parts and two parts are transmitted in one burst. To this end, four bursts are needed for transmitting the whole data frame, which means that the transmission of one data frame lasts approximately 20 ms. On the other hand, each part can be transmitted in a burst of its own, wherein bits of two different data frames can be transmitted in one burst. FIG. 1a shows a fifty-two multiframe which is used for transmitting data packets and is thus composed of fifty-two TDMA frames. Twelve data frames can be transmitted in such a fifty-two multiframe (B0 to B11). In turn, FIG. 1b shows a twenty-six multiframe which is used for transmitting speech and is thus composed of twenty-six TDMA frames.
In the exemplified GSM system, the length of one timeslot is approximately 577 μs and the length of the TDMA frame (8 timeslots) is thus approximately 4.615 ms. Interlocking data frames e.g. into four TDMA frames involves that the transmission interval is approximately 20 ms.
Among other things, one problem present in such faster connections is, that the power consumption of a wireless communication device increases the more often the wireless communication device is transmitting. Therefore, an increase in the number of transmission timeslots allocated for the wireless communication device will increase the power consumption of the wireless communication device. This increased power consumption shortens the operation time of the battery, wherein the battery has to be charged more often. Additionally, the power will partially turn into heat in the wireless communication device, wherein more attention is to be directed to cooling, among other things, for the reason that an increase in the operation temperature will increase the probability of damaging the components of the wireless communication device and shortening the life of the wireless communication device.
One solution to the previously described problem would be to reduce the transmission power (power grade) of the wireless communication device, e.g. from 2 W to 0.8 W. However, in practice this would involve that the strength of the signal transmitted by the wireless communication device decreases, which can limit the distance between the wireless communication device and the base station. Thus, the connection can be even disconnected, particularly at border areas of a cell.
Another solution would be to reduce the number of transmission timeslots e.g. to one timeslot per frame. At the same time, this involves that the communication speed from the wireless communication device to the base station (uplink) is decreased, which is not always advisable. On the other hand, reducing the number of transmission timeslots enables using higher transmission power for one timeslot. This will involve that the data transmitted from the wireless communication device to the base station is more likely to be duly received and there will be less need for re-transmissions, wherein the transmission speed can even be improved.
International patent application WO 00/31990 introduces a power saving system in which the temperature of the wireless communication device is monitored by using a temperature sensor. The temperature shown by the temperature sensor is compared to a threshold value, and if the temperature exceeds this threshold value, the transmission power or the transmission speed (the number of transmission timeslots per time unit) is reduced. In some situations it may even be necessary to cut off the connection, if the reduced transmission speed is insufficient for the application transmitting information from the wireless communication device. This kind of arrangement requires also that the temperature sensor is located in a suitable place in the wireless communication device, as well as means for processing the data provided by said temperature sensor. However, the temperature may in some other location exceed the temperature shown by the temperature sensor, wherein the measuring result is not necessarily reliable.
Increasing power consumption can also be problematic because wireless communication devices may comprise other transmitters than the one used for communication with a mobile communication network. The wireless communication device can e.g. have short range communication means, such as a Bluetooth™ radio transceiver. These communication means can operate simultaneously with the transmitter transmitting to the mobile communication network, which can also increase the power consumption of the wireless communication device.